In a mass storage device, information is written onto moving magnetic or optical media as data coded into a series of bits. The bits are then "written" onto the media in the form of magnetically or optically detectable pulse changes in the medium by means of a write head.
For retrieving the recorded information, the data rae read by a read head and decoded by means of a read channel into the original sequence of bits.
The purpose of the read channel is to process the weak recorded signal read by the read head so that a subsequent recovery of a digital signal representing the recorded information is possible. For this purpose, the recorded signal is customarily first prepared by amplification, filtering, compensation for linear distortion, amplitude control or the like. Thereafter, in order to determine the maximum amplitude of the recorded signal, the signal is differentiated and compared in a threshold value detector with a predetermined threshold value in order to recover a digital signal. The still coded digital signal is then decoded in a decoder to recover the original bit sequence.
In the known techniques, exceeding the threshold value is generally used as the determining criterion as to whether a pulse change has taken place or not. The disadvantages of the known techniques, which show themselves as falsely detected signal pulses become obvious when the signal to noise ratio decreases.
Since the error rate of data decoding varies with the signal to noise ratio, with increasing storage density, the criterion for recovery a digital from the recorded signal become more critical. Current storage devices show that if the signal to noise ratio decreases below the range of about 20 to 26 dB there is a substantial increase in the error rate. These errors can, within limits, be corrected with error correction procedures. However, this requires a substantial expenditure of time in the entire storage device and accordingly reduces the maximum achievable data throughput in comparison to the theoretically possible data throughput.